Report on Terrestrial Magnetism. 317 



it will be proper to lay before the Council a full account of the 

 communication itself. In this letter M. deHumboldt developes 

 a plan for the observation of the Phenomena of Terrestrial Mag- 

 netism worthy of the great and philosophic mind whence it has 

 emanated, and one from which may be anticipated the establish- 

 ment of the theory of these phenomena. 



After his return from the equinoctial regions of America, M. 

 de Humboldt, in the years 1806 and 1807, entered upon a care- 

 ful and minute examination of the course of the diurnal variation 

 of the needle. He was struck, he informs us, in verifying- the 

 ordinary regularity of the nocturnal period, with the frequency of 

 perturbations, and, above all, of those oscillations, exceeding the 

 divisions of his scale, which were repeated frequently at the same 

 hours before sunrise. These eccentricities of the needle, of 

 which a certain periodicity has been confirmed by M. Kupffer, 

 appeared to M. de Humboldt to be the effect of a reaction from 

 the interior towards the surface of the globe he ventures to say of 

 " magnetic storms'' which indicated a rapid change of tension. 

 From that time he was anxious to establish to the east and to 

 the west of the meridian of Berlin, apparatus similar to his own, 

 in order to obtain corresponding observations made at great dis- 

 tances at the same hours, but was for a long period prevented 

 putting his plan into execution by the disturbed state of Ger- 

 many and his departure for France. 



The Baron de Humboldt and MM. Arago and Kupffer ha- 

 ving, by the co-operation of many zealous observers, succeeded in 

 establishing permanent magnetic stations extending from Paris 

 to China, M. de Humboldt solicits, through his Royal Highness 

 the President, the powerful influence of the Royal Society in 

 extending the plan, by the establishment of new stations. The 

 plan which he proposes, and which has been successfully carried 

 into execution over a large portion of the north-eastern conti- 

 nent, is, that magnetical observation:*, whether of the direction 

 of the horizontal and inclined needles, or for the determination 

 of the variations of the magnetic force, should be made simultane- 

 ously at all stations, at short intervals of time, for a certain num- 

 ber of hours and at fixed periods of the year, precisely similar 

 to the plan which has been recommended and adopted by Sir 



